1. Technical Field
The present invention relates to printed circuit board connectors. In particular, the present invention relates to printed circuit board edge connectors.
2. Description of the Related Art
Printed circuit (PC) cards or printed circuit boards (PCBs) have traditionally provided mechanical support for electronic components while providing electrical interconnections of these components. One means to connect a printed circuit board to other electronic circuitry is to provide a printed circuit board edge connection assembly using a series of contact pads fabricated and arranged in a row at the edge of the printed circuit board. These edge contact pads are electrically connected to the components upon the printed circuit board. Further, to connect the circuitry on a printed circuit board to other electronic system components, a printed circuit board socket connector is provided that receives a printed circuit board and its edge contact pads. This printed circuit board socket contains several metal contact springs positioned in the socket such that the insertion of the printed circuit board into the socket results in individual metal contact springs resting upon the surface of the printed circuit board and, specifically, contacting an individual one of the edge contact pads.
A typical prior art printed circuit board edge connection assembly is shown in part in FIG. 1A. In FIG. 1A, four contact spring connectors 104, 106, 108, and 110 are arranged to be placed upon four printed circuit board edge contact pads 112, 114, 116, and 118 which are upon the top surface 120 of the printed circuit board 100. Since the four contact spring connectors 104, 106, 108, and 110 are part of a printed circuit board edge connection socket (not shown) the contact spring connectors 104, 106, 108, and 110 are positioned when the printed circuit board 100 is inserted into the socket so that the contact spring connectors 104, 106, 108, and 110 rest on top of the printed circuit board edge contact pads 112, 114, 116, and 118 to form an electrical connection.
The contact spring connectors 104, 106, 108, and 110 and the printed circuit board edge contact pads 112, 114, 116, and 118 are typically plated with gold over nickel to minimize contact surface corrosion. One of the main problems with the contact edge connector assembly shown is the excessive wear of the contact spring connector bottom surface when mating the printed circuit board to the printed circuit card edge connector socket (not shown) containing the contact spring connector. Referring to FIG. 1B, a cross section view of FIG. 1A, as the printed circuit board 100 is inserted into the socket (not shown), the contact spring connector 110 first contacts the printed circuit board front edge 102 and, as the insertion continues, rides over the top of this edge 102 and then contacts the front edge of the printed circuit edge contact pad 118 upon the printed circuit board surface 120. Typically, the printed circuit board edge 102 of printed circuit board 100 is formed with a router bit resulting in a sharp edge having exposed glass fibers from the glassy epoxy laminate material of the printed circuit board. Continuing the insertion, the contact spring connector 110 rides over the edge of the contact pad 118 and onto the surface of the contact pad 118. During this insertion, the contact spring connector 110 has traveled over the printed circuit board edge 102 and then the contact pad 118 edge which can result in wearing of the contact spring connector surface plating. Many printed circuit board insertions will remove the gold and nickel plating leaving the contact spring connectors susceptible to corrosion which may cause reliability problems. In addition, other problems with the printed circuit board edge connection assembly of FIGS. 1A and 1B include the amount of force required to mate the printed circuit board with the printed circuit board socket, stubbing that may result in damage to the spring connector and misalignment of the spring connector during insertion that may result in shorting between across adjacent contact pads.
Alternate solutions to these problems are shown in FIGS. 2, 3, 4 and 5. In FIG. 2, a cross section view, a printed circuit board 200 includes a front edge 202 having chamfer edges 206 and 208. The chamfer edges 206 may reduce the edge scraping wear of a contact spring connector 210 as it engages the printed circuit board 200 and comes to rest upon the top of the contact pad 218. However this arrangement still results in wearing of the bottom surface of the contact spring connector 210 with little to no effect on the insertion force required.
FIG. 3 is another embodiment shown in cross section illustrating a printed circuit board 300 having a TEFLON (a trademark of DuPont) edge 302 which provides an inclined surface 306 to receive the contact spring connector 310 to reduce the printed circuit board edge connection insertion force and reduce wear on the spring contact connector 310. However, the addition of the Teflon cap 302 increases the complexity of the printed circuit board assembly resulting in an increase in manufacturing cost.
A further embodiment shown in cross section in FIG. 4 illustrates a printed circuit board 400 having a rounded front edge 402 with a surface coating of epoxy. Surface 406 is the first contact point for the contact spring connector 410 as the printed circuit board 400 is inserted. The use of the epoxy coating on the printed circuit board edge 402 reduces the insertion force required to assemble the printed circuit board into the printed circuit board edge connector socket and reduces wear on the contact spring connectors. However, the step of adding the epoxy coating exposes the contact pads, such as contact pad 418, to contamination and this step also adds complexity to the printed circuit board assembly resulting in increased manufacturing costs.
Another embodiment is illustrated in FIG. 5 which is a cross section view of a printed circuit board 500 including a continuous flex circuit film 504 completely covering a thermally conductive stiffener which is typically a metal plate. A contact pad 502 is fabricated on the surface of the flexible circuit film 504. As the printed circuit board 500 is inserted into a printed circuit board edge connector socket (not shown) a contact spring connector 510 will first encounter the flex circuit film before it contacts the edge contact pad 502 riding up on the contact pad 502 edge indicated as 506. It should be understood for this embodiment, the flexible circuit film covers the entire printed circuit board. As such, only the flexible film surface is available for the mounting of electrical components electronic components and wiring. In other words, it is a two layer printed circuit board assembly. However the continuous flexible circuit film does reduce the insertion force required to mat the printed circuit board with a printed circuit board edge connector socket and reduces wear on the contact spring connectors during the insertion process. Providing a continuous flexible circuit film bonding onto a conductive stiffener increases the cost of the printed circuit board manufacture and decreases the versatility of the printed circuit board assembly since only two layers are available for all electronic component wiring.